Levels 5–8: Intermediate

Contents

• Level 5 — Process Management
• Level 6 — Text Processing
• Level 7 — Networking
• Level 8 — Shell Scripting Basics
  • Script Structure

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Level 5 — Process Management

Linux runs hundreds of processes at once. You need to control them.

Command	What it does
ps aux	List ALL running processes (all users)
ps aux \| grep nginx	Find a process by name
kill PID	Send SIGTERM (polite stop) to process
kill -9 PID	Send SIGKILL (force stop, cannot be ignored)
jobs	List background jobs in current shell
bg	Resume a stopped job in the background
fg	Bring background job to foreground
command &	Start a command directly in the background
top	Live process viewer (q to quit)
lsof -i :80	Show what process is using port 80

ps aux columns:

Column	Meaning
USER	Owner of the process
PID	Process ID
%CPU	CPU usage
%MEM	Memory usage
STAT	State: R=running, S=sleeping, Z=zombie, T=stopped
COMMAND	The command that was run

kill -9 cannot be caught or ignored by the process — it terminates immediately with no cleanup. Use it only when a normal kill has failed.

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Level 6 — Text Processing

Transform data at machine speed without a GUI.

Command	What it does	Example
cut -d: -f1 file	Extract field 1, delimiter :	cut -d: -f1 /etc/passwd
awk '{print $1}' file	Print first field of each line	Column extraction
awk '$2 > 50' file	Filter rows by condition	Numeric comparisons
awk 'END{print sum}' file	Run code after all lines	Totals and summaries
sed 's/old/new/g' file	Replace all occurrences	Find and replace
sed '/pattern/d' file	Delete lines matching pattern	Strip noise
sed -n '2,5p' file	Print lines 2–5 only	Extract a range
head -n 10 file	First 10 lines	Preview
tail -n 10 file	Last 10 lines	Recent entries
tr 'a-z' 'A-Z'	Translate/convert characters	Case conversion

awk built-in variables:

Variable	Meaning
$0	Entire line
$1, $2…	Fields 1, 2…
$NF	Last field
NR	Current line number
NF	Number of fields on current line
FS	Field separator (default: whitespace)

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Level 7 — Networking

Linux is the internet. Know how to interact with it.

Command	What it does
ping -c 4 host	Test connectivity, 4 packets
curl url	HTTP GET request, print response
curl -o file url	Download URL to file
curl -X POST -H 'Content-Type: application/json' -d '{...}' url	POST JSON
wget url	Download file (saves automatically)
ssh user@host	Connect to remote server
ssh -i ~/.ssh/key user@host	Connect with specific key
scp file user@host:/path/	Copy file to remote server
ss -tlnp	Show listening TCP ports with process names
netstat -tlnp	Same (older tool, still widely available)

curl common flags:

Flag	Meaning
-o file	Save output to file
-O	Save with remote filename
-L	Follow redirects
-s	Silent mode (no progress)
-I	HEAD request only (headers)
-u user:pass	Basic authentication
-k	Skip SSL certificate verification

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Level 8 — Shell Scripting Basics

Automate everything. Scripts are commands in a file.

Script Structure

#!/bin/bash
# Every script starts with a shebang — tells the OS which interpreter to use

NAME="BashQuest"
echo "Hello from $NAME"

Concept	Syntax	Example
Variable	NAME=value	NAME=Tony
Use variable	$NAME or ${NAME}	echo $NAME
Command output	$(command)	NOW=$(date)
If statement	if [ cond ]; then ... fi	if [ -f file.txt ]; then
For loop	for i in list; do ... done	for i in 1 2 3; do echo $i; done
While loop	while condition; do ... done	while true; do sleep 1; done
Function	name() { ... }	greet() { echo "Hi $1"; }

File test operators:

Test	True when
-f file	File exists
-d dir	Directory exists
-e path	File or directory exists
-z "$var"	Variable is empty
-n "$var"	Variable is not empty

Always quote variables in tests: [ -f "$FILE" ] not [ -f $FILE ]. An unquoted variable with spaces breaks the test.